1. Field of the Invention
The present invention generally relates to the manufacture of semiconductor devices and circuits and, more particularly, to a process using chlorine (Cl.sub.2) and hydrogen chloride (HCl) gases for sub-micron metal etching in a reactive ion etch (RIE).
2. Background Description
Traditional RIE processes use boron chloride (BCl.sub.3) and Cl.sub.2 chemistries with relatively high source power (500 Watts or more). The historic reason for the use of BCl.sub.3 is that the gas, in addition to functioning as an etchant, also performs the function of an oxygen (O.sub.2) scavenger in the reactor. In the presence of O.sub.2 (or moisture), BCl.sub.3 dissociates into B.sub.2 O.sub.3 and Cl.sub.2. The drawback of BCl.sub.3 is that B.sub.2 O.sub.3 is a source of particulates in the reactor.
For modern high ion density reactors which operate below 15 milliTorr (mT), O.sub.2 or moisture scavenging agents are not needed, thus this property of BCl.sub.3 is not utilized. Also, high power processes using this approach have certain problems. One of these problems is a high rate of resist erosion and loss of a substantial amount of substrate dielectric, generally silicon dioxide (SiO.sub.2). Furthermore, this type of process is more prone to corrosion, and so-called "mouse bites" which are characterized as a lateral loss of Al--Cu (0.5% copper in aluminum) which is not desirable. FIG. 1 is a scanning electron micrograph (SEM) of a "mousebite" and FIG. 2 is an SEM showing an example of corrosion. Corrosion takes place as a result of Cl.sub.2 attack on side walls (SW) of etched metal lines in the presence of H.sub.2 O. A "mousebite" is a form of corrosion which occurs as a result of lack of SW passivation.
In conventional metal etching, the ion density is assumed to control the side wall profile, and high source power (on the order of 500 Watts or more) is a common way of achieving high ion density. U.S. Pat. No. 5,387,556 to Xiaobing et al. discloses a process of etching of aluminum and aluminum alloys using HCl, Cl-containing etchant and nitrogen (N.sub.2) that employs conventional capacitatively coupled plasma technology. In capacitatively coupled plasmas, power is provided at either the top or bottom of the chamber, and the plasma acts as a capacitor. Higher power and higher pressures in the chamber leads to higher ion densities. Unfortunately, higher power and pressure leads to greater erosion of the side wall and increased susceptibility to charge damage and corrosion. In addition, Xiaobing et al. utilizes a magnetic field during etching which is not preferable due to the possible exposure to charge damage to the semiconductor devices on the wafer. U.S. Pat. No. 3,994,793 to Harvilchuck et al. also discloses RIE of aluminum utilizing halogen containing etchants, and particularly chlorine, bromine, hydrochloric acid, and carbon tetrachloride. Harvilchuck also teaches the use of a capacitatively coupled plasma, and relies on "sputtering" or "bombarding" of the layer to be etched with chlorine or bromine ions. U.S. Pat. No. 5,223,085 to Kawai discusses anisotropic etching of a layer using plasmas created from hydrogen halides. In Kawai et al., the plasma is created from an electron cyclotron resonance (ECR) source, wherein high power inputs are used to transfer energy to electrons in the plasma by a resonance matching scheme.